The present invention relates to a method and apparatus for feeding solids into the bed of a fluidized bed combustion furnace and in particular to a method and apparatus for feeding fine feed solids so as to extend the residence time of the fine feed solids in the fluidized bed.
In present fluidized bed combustion systems, the feed solids are typically discharged through nozzles or openings located in or above the fluidized bed. Combustion air serves as fluidizing air and is supplied to an air plenum located beneath the fluidized bed. The fluidizing air passes upward from the air plenum into the fluidized bed through a perforated bed support plate at a flow rate sufficiently high to fluidize the feed solids within the fluidized bed. The feed solids are comprised of sulfur oxide sorbent and sulfur containing carbonaceous fuel. Combustion occurs in the fluidized bed and in the freeboard region above the bed. The combustion flue gases exit the freeboard region through the top of the fluidized bed furnace.
In a typical fluidized bed pneumatic transport feed system, discharge nozzles are located near the bottom of the fluidized bed above the perforated bed support plate. The feed solids and pneumatic transport air are released into the bed at the discharge nozzles. The pneumatic transport air passes directly upward through the bed from the discharge nozzles, resulting in locally increased gas velocity and subsequent entrainment of fine feed solids. The fine feed solids are carried upwardly through the fluidized bed and elutriated into the freeboard region above the bed without thoroughly mixing with the fluidized feed solids within the bed. Rapid elutriation of the fine feed solids lowers the residence time of the fine feed solids in the fluidized bed. Due to inadequate mixing and reduced residence time, the fine feed solids are not completely reacted in the fluidized bed.
More thorough mixing of the fine feed solids with the fluidizing air and with the coarser feed solids in the fluidized bed would provide a longer residence time of the fine feed solids in the fluidized bed and in turn facilitate a more complete reaction.